a(n) = 5^n - 5*n.

Terms

a(0) =1a(1) =0a(2) =15a(3) =110a(4) =605a(5) =3100a(6) =15595a(7) =78090a(8) =390585a(9) =1953080a(10) =9765575a(11) =48828070a(12) =244140565a(13) =1220703060a(14) =6103515555a(15) =30517578050a(16) =152587890545a(17) =762939453040